High temperature strength columbium base alloys



section.

United States Patent 3,297,438 HIGH TEMPERATURE STRENGTH COLUMBIUM BASE ALLOYS Elihu F. Bradley, West Hartford, Conn., Robert I. Jalfee and Dean N. Williams, Columbus, and Edwin S. Bartlett, Worthington, Ohio, assignors, by direct and mesne assignments, to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware No Drawing. Continuation of application Ser. No. 65,962, Oct. 31, 1960. This application Apr. 6, 1964, Ser. No. 358,972

8 Claims. (Cl. 75-174) This application is a continuation of our application Serial No. 65,962, filed October 31, 1960, and now abandoned.

The present invention relates to novel columbium base alloys and more particularly to improved alloys of columbium with two or more of the elements selected from the group consisting of tantalum, tungsten, and molybdenum which possess excellent strength properties both short time and in stress-rupture when used in high temperature environments. These alloys also exhibit high recrystallization temperatures, some room temperature ductility, fabricability, and good resistance to elevated temperature contamination and oxidation by columbiumbase alloy standards.

The principal limitation in gas-turbine technology today is the maximum turbine inlet temperature. The turbine inlet temperature is in turn limited by the temperature which the turbine vanes and blades are able to withstand Without danger of failure. The best available high temperature alloys in the prior art are nickel and cobalt base superalloys, but critical structural components such as turbine vanes and blades constructed from such alloys are limited to maximum operating temperatures of between 1600 and 1900 F.

Among the technically more important physical qualities of columbium as an alloy base are its high melting temperature (4380 F.) and its low neutron-capture cross- Columbium is therefore potentially useful for fast aircraft and space flight vehicles and in nuclear reactors.

For many years it has been generally known that the high temperature strength properties of metals are closely related to their melting points. Thus, metals having a high melting point also tend to have high temperature strength potentials. V

The need for structural materials for service at temperatures in excess. of those attainable with present materials of construction has stimulated interest in the refractory metals. Until about 1957, molybdenum was considered the chief prospect for such uses. Molybdenum, however, at the high temperature service conditions needed, oxidizes at a catastrophic rate principally because molybdenum oxide is volatile at elevated temperatures. Further, no satisfactory way of overcoming the propensity of molybdenum to oxidize has been found. Because of the very great problems with molybdenum, interest has recently shifted to columbium as an alloy base for high temperature service.

Columbium is inherently a soft, ductile, readily fabricable material. Although its melting temperature is about 4380" F., pure columbium becomes too weak for structural use at temperature above about 1200 F. Columbium is a very reactive metal in that it dissolves large quantities of oxygen, and probably nitrogen, on exposure to atmospheres containing even small amounts of these elements at modest temperatures. Although columbium suffers from oxidation, its oxide does not volatize, and it is not subject to the catastrophic oxidation mechanism of molybdenum. It. is thus potentially possible to. local- Patented Jan. 10, 1967 ice ize oxygen attack on columbium by coating the metal. Further advantages offered by columbium base alloys as compared with molybdenum base alloys are that columbium alloys are relatively more ductile and workable at low temperatures and columbium has a lower density than molybdenum. The strength and oxidation resistance of pure columbium can be vastly improved by the addition of alloying elements.

Until recent years, estimated ore reserves of columbium were so small that there was only a mild interest in columbium base alloys. With the discovery of substantial ore bodies, however, the potential availability of columbium is so great that scarcity is no longer a restriction on its use.

In view of the foregoing, it is a primary object of this invention to provide a columbium base alloy which has excellent high temperature strength, both in short time tensile and in stress-rupture, which is fabricable, and which has resistance to oxidation at elevated temperatures by columbium base alloy standards.

Additional objects of this invention are to provide a high temperature strength columbium base alloy which is ductile at room temperature, which possess excellent resistance to corrosion, which has improved elevated temperature resistance to both oxidation and contamination,

which has both high tensile and stress-rupture strength at elevated temperatures, such as 2200 F., which has good fabricability characteristics, which has improved elevated temperature oxidation and contamination resistance without the use of titanium which is detrimental to elevated temperature strength, which has a low brittle to ductile transition temperature, which has improved resistance to scaling, and which has a low rate of contamination by columbium base alloy standards.

A further object of this invention is to provide a novel high temperature strength columbium base alloy which has a high recrystallization temperature. The temperature required to initiate recrystallization in the alloys of this invention is in the range of 2400-2500 F., and the temperature required to complete recrystallization is in the range of 2700-2800 F.

It is another object of this invention to provide a novel columbium base alloy which when used for turbine vanes and blades will permit raising the turbine inlet temperature from the present limit of 1600 to 1900 F. with nickel and cobalt super-alloys to a temperature of 2200 F. It

is acknowledged in the art of gas turbine technology that even an increase of as little as 100 F. would be extremely significant. In a jet engine application an increase in turbine inlet temperature would permit an increase in the total thrust of the engine at a better eficiency. The alloys of the present invention promise an increase of at least 3 00" F., an increase of dramatic importance.

Another object of this invention is to provide a novel high temperature strength columbium base alloy which is solid solution strengthened by either tungsten or molybdenum.

A further object of this invention is to provide a columbium base alloy which contains tantalum in addition to tungsten or molybdenum, the tantalum serving to lower the brittle to ductile transition temperature of the alloy which is otherwise adversely aifected by the addition of tungsten or molybdenum to pure columbium.

It is a still further object of this invention to provide a novel columbium base alloy which is solid solution strengthened by tungsten, molybdenum, or both and which is subjected to second-phase hardening by the addition of hafnium. The hafnium combines with a small amount of carbon or oxygen present in the columbium to form compounds which produce second-phase hardening characteristics.

The novel alloy of this invention contains from about 50% to 80% by weight of columbium, from to 20% by weight in the aggregate of at least one element selected from the group consisting of tungsten and molybdenum, the content of molybdenum being a maximum of by weight of the alloy, and from to 40% by weight of tantalum in combination with the elements set forth above, the alloy may additionally contain from 0.5 to 7% by weight of one or more of the elements hafnium and vanadium to give to the alloy certain desired properties such as additional strength at high temperatures, and resistance to scaling and contamination.

The novel alloys of this invention can be prepared by using known melting and casting techniques. To insure homogeneity, it is desirable to use multiple melting. Individual melts can be melt-cast together, and the melt may be permitted to cool and solidify into a predetermined shape.

In operation the melting can be achieved by either an induction-type furnace or an arc furnace using either consumable on non-consumable electrodes. An arc melting furnace using a chilled copper crucible has been used advantageously.

Regardless of the type of furnace used, great care must be taken in the melting and casting operation to protect the molten metal from normal atmospheric contamination by oxygen, nitrogen, and other gases which may be present. This problem can be avoided by conducting the melting operation in a controlled atmosphere.

In preparing the metals for the charge almost any appropriate form of the metals may be used, such as shot, granular, powder, or wire. Ideally, the metals should be of the highest purity commercially obtainable. The alloy obtained will provide a workable metal which can be used in high temperature applications and retain high strength at temperatures well in excess of those to which present day high temperature alloys can be subjected.

The alloys of this invention have good strength properties at temperatures, such as 2200 F., at which the prior art high temperature alloys lose strength significantly.

In forming the novel alloys of this invention, the addition of tungsten, or molybdenum, or both to pure columbium provides solid solution strengthening of the columbium at high temperatures. Unfortunately, both tungsten and molybdenum adversely affect the room temperature ductility and fabricability of columbium. The addition of tantalum, however, tends to counteract the adverse effects of tungsten and molybdenum on ductility. The tantalum in the alloy lowers the brittle to ductile transition temperature and gives the alloy better fabricability and room temperature ductility.

The addition of hafnium to pure columbium increases the short time tensile strength of the columbium, but since it also lowers the recrystallization temperature, it may have an adverse effect on the high temperature stressrupture properties of columbium. When hafnium is added to a columbium alloy of high recrystallization temperature, such as the alloys of this invention, however,

the stress-rupture strength of the alloyat 2200 F. is

A mixture of by weight of columbium, 20% by weight of tantalum, 15% by weight of tungsten, and 5% by weight of molybdenum was introduced as a charge into a watercooled, copper crucible of a non-consumable electrode type melting furnace using tungsten electrodes. Prior to melting the furnace chamber was evacuated three times and then back-filled with helium to provide a sufliciently inert air-free controlled atmosphere. Each charge was melted a minimum of eight times, with the total number of melts needed for homogeneity being determined by operator observation and radiography. When the charge had been melted for the last time, the furnace was turned off and the melt was allowed to cool in the helium atmosphere, discharged from the crucible and tested as described below.

The testing procedure which was used for this example was also used for all the other examples given. The testing procedures used was as follows: An arc-cast 2 x x button ingot was machined flat on the ends and sides to 2% x x A The hardness of the machined button was then measured.

After machining, the arc-cast ingot was encased in thin sheets of molybdenum. The sheathed ingots were inserted in tight fitting holes machined in a boiler plate yoke assembly. Stainless steel cover plates were welded on the top and bottom of the yoke and a low carbon steel evacuation tube was also Welded in place. The gas tight yoke cover assembly was then evacuated to approximately microns of mercury at 1800 F. The assembly was then sealed by forging. Finally the assembly was rolled at 1800 F. to an overall thickness of Ms".

After rolling, the alloy strips, now measuring about 8 x A: x .090", were recovered. The strips were then conditioned by grinding and pickling, and their hardness was measured. Qualitative ratings of fabricability were made by visual observation. The fabricability of this alloy at 1800 F. was good, i.e., the only flaws detectable were very fine surface, edge, or end cracks.

The alloy was then recrystallized for one-half hour at 3000 F. and its hardness was measured. Next a check for final rolling temperature was made using small samples. Tests were made at room temperature, 500 F., and 1000 F. or 1800 F. Finally the strips were rolled to .030" thickness at the lowest permissible temperature and the hardness was again measured. For this particular alloy a good fabrication temperature was 1800 F.

Coupons of'the alloy were machined and stress relieved for one-half hour at 2200 F. in an argon atmosphere. These coupons were then tested to failure in tension at both room temperature and at 2200 F.

The room temperature tests were performed on a universal testing machine at cross-head speeds of 0.005" per minute to the yield load, and at a rate of 0.05" per minute to failure.

The 2200 F. tests were performed in a vacuum of 0.001 mm. of mercury in a creep-test rack. The specimens were wrapped with tantalum foil for gettering purposes. The approxirnate time of exposure to temperatures in excess of 1800 F. was one-half hour. The strain rate was maintained at about 0.1" per minute throughout the test period. In these tests the ultimate tensile strength of this alloy at room temperature was 141,000 p.s.i. and its tensile strength at 2200 F. was 54,000 p.s.i.

Stress-rupture tests were performed on specimens of the alloy at 2200 F. in a vacuum of about 0.0001 mm. of mercury. For these tests the specimens were again wrapped in tantalum foil. The stress-rupture strength of this alloy at 2200 F. was 31,000 p.s.i. for ten hours.

The one hour recrystallization temperatures for this alloy were 2450 F. to initiate recrystallization and 2800 F. to complete recrystallization.

For oxidation tests a specimen was exposed to a staticair-atmosphere at 2200 F. for one-half hour. Specimens were prepared by grinding to a uniform finish with a 240-grit abrasive. After carefully weighing and measuring all critical components, the specimens were exposed in prefired porcelain crucibles in such a manner that maximum surface exposure was achieved. After exposure each specimen was evaluated by measuring weight gained (metal plus oxide), weight loss (metal plus adherent oxide), metallographic determination of metal loss, and microhardness traverse to determine the depth of contamination. The depth of contamination was arbitrarily set at the distance normal to the metal surface after exposure over which the hardness increases no less than 50 VHN over the pre-exposure base-metal hardness. For this alloy the weight gain (oxide plus scale) in mg./cm. was 19 and the weight loss (brushed) was 58. The metals'loss in mils per side was 1, the depth of contamination in mils was greater than 14 and the approximate change in hardness measured in VHN at the surface was 230 and at the center was 145. The distance from the surface below which hardness was constant in mils was 6.

The room temperature ductility of this alloy was evaluated by its room temperature tensile elongation in percent, by-bendtests of stress-relieved specimens, and by bend tests on specimens strained at 2200 F. This alloy had a room temperature tensile elongation of 3% and an overall rating of marginal room temperature ductility.

Example 2 An ingot of a columbium, tantalum, tungsten, molybdenum, hafnium metal alloy composition containing by weight 20% of tantalum, 10% of tungsten, 5% of molybdenum, 5% of hafnium, and the balance of columbium was prepared as described in Example 1.

The tests of this alloy were performed in the same manner as the tests set forth in Example 1. The ultimate tensile strength of this alloy at room'temperature was 110,000 p.s.i. Its ultimate tensile strength at 2200 F. was 67,000 p.s.i., and its hour stress rupture strength at 2200 F. was 30,000 p.s.i. The temperature required for good fabricability of this alloy was 1000 F. and its room temperature ductility was very good. The temperature required to initiate recrystallization in this alloy was 2300" F. and the temperature required to complete recrystallization was 2700 F.

The results of oxidation testingon this alloy were as follows: weight gain in mg./cm. (oxide plus scale) was 24 and weight loss (brushed) was 59; metal loss in mils per side was 0; the depth of contamination in mils was 14; the approximate change in hardness measured in VHN at the surface was 500 and at the center was 0; the distance from the surface below which hardness was constant in mils was 14.

Example 3 An ingot of a columbium-tantalum-tungsten-molybdenum alloy composition containing by weight of tantalum, 10% of tungsten, 7.5% of molybdenum and the balance of columbium was prepared and tested as described in Example 1.

The are melted alloy of this example had an ultimate tensile strength at room temperature of 126,000 p.s.i. The ultimate tensile strength of this alloy was 54,000 p.s.i. at 2200 F., and its 10 hour rupture strength at 2200 F. was 27,000 p.s.i. A temperature of 1800 F. was required for good fabricability, and the alloy exhibited good room temperature ductility. A temperature of 2350 F. was required to initiate recrystallization and a temperature of 2600 F. was required to complete recrystallization.

Oxidation tests on this alloy yielded the following results: weight gain in mg./cm. (oxide plus scale) was 24; weight loss (brushed) in mg./cm. was 66; metal loss in mils per side was 2; depth of contamination in mils was 18; the approximate change in hardness measured in VHN at the surface was 155 and at the center was 10; the

distance from the surface below which hardness was constant inmils Was24.

Example 4 An ingot of a columbian-tantalum-tungsten metal alloy composition containing by weight 20% of tantalum, 20% of tungsten and the balance of columbium was prepared and tested as described in Example 1.

The are melted alloy of this example had an ultimate tensile strength at room temperature of 93,000 p.s.i. This alloy had an ultimate tensile strength at 2200 F. of 49,000 p.s.i. and a 10 hour stress-rupture strength at 2200 F. of 29,000 p.s.i. A temperature of 1800 F. was required for good fabricability. This alloy was essentially-brittle at room temperature. A temperature of 2400 F. was required to initiate recrystallization and a temperature of 2750 F. was required to complete recrystallization. Oxidation tests of this alloy were not made.

Example 5 An ingot of a columbium tantalum molybdenum metal alloy composition containing by weight 20% of tantalum, 15% of molybdenum, and the balance of columbium, was prepared and tested as described in Example 1.

The are melted alloy of this example had an ultimate tensile strength at 2200 F. of 56,000 p.s.i. and a 10 hour stress-rupture strength at 2200 F. of 27,000 p.s.i. A temperature of 1800 .F. was required for good fabricability. The room temperature ductility of this alloy was good. Oxidation tests were not made.

Example 6 An ingot of a columbium tantalum tungsten vanadium metal alloy composition containing by weight 20% of tantalum, 10% of tungsten, 5% of vanadium and the balance of columbium was prepared and tested as described in Example 1.

The are melted alloy of this example had an ultimate tensile strength at 2200 F. of 66,000 p.s.i. and a 10 hour stress-rupture strength at 2200 F. of 10,000 p.s.i. A temperature of 1800 F. was required for good fabricability. This alloy showed good ductility at room temperature. A temperature of 2000 F. was required to initiate recrystallization and a temperature of 2400 F. was required to complete recrystallization.

Oxidation tests of this alloy yielded the following results: weight gain (oxide plus scale) in mg./cm. was 18; weight loss (brushed) in mg./cm. was 28; metal loss in mils per side was 1; the depth of contamination in mils was 26; the approximate change in hardness measured in VHN at the surface was 425 and at the center was -25; the distance from the surface below which hardness was constant measured in mils was 26.

Example 7 An ingot of a columbium tantalum tungsten molybdenum vanadium metal alloy composition containing by weight 20% of tantalum, 10% of tungsten, 5% of molyb denum, 3% of vanadium, and the balance of columbium was prepared and tested as described in Example 1.

The are melted alloy of this example required a temperature of 1 800 F. for good fabricability. Data on tensile strength and stress rupture strength are not available.

Oxidation tests of this alloy yielded the following results: weight gain (oxide plus scale) in mg./cm. was 9; weight loss (brushed) in mg./cm. was 29; metal loss in mils per side was -1; depth of contamination in mils was 28; the approximate change in hardness in VHN at the surface was 820 and at the center was 30; the distance from the surface below which hardness was constant measured in mils was 28.

The purity of the columbium used in these alloys was of the highest obtainable. The impurities contained in the columbium used did not exceed 500 parts per million.

The present invention in its broader aspects is not limited to the specific compositions and examples de scribed, but also includes within the scope of the accompanying claims any departures made from such compo sitions and examples which do not sacrifice their chief advantages.

What is claimed is:

1. A ductile colu-mbium-base alloy having good stressrupture strength at high temperatures and consisting essentially of about 20% by weight of tantalum, about 15% by weight of tungsten, about by weight of molybdenum, and balance essentially columbium.

2. A ductile columbium-base alloy having good stressrupture strength at high temperatures and consisting essentially of about 20% by weight of tantalum, about 10% by weight of tungsten, about 5% by weight of molybdenum, about 5% by weight of hafnium, and balance essentially columbium.

3. A ductile columbium-base alloy having good stressrupture strength at high temperatures and consisting essentially of about 20% by weight of tantalum, about 10% by weight of tungsten, about 5% by weight of molybdenum, about 3% by weight of vanadium, and balance essentially columbium.

4. A ductile columbium-base alloy having good stressrupture strength at high temperatures and consisting essentially of about 20% by weight of tantalum, about 10% by weight of tungsten, about 7.5% by weight of molybdenum, and balance essentially columbium.

5. A ductile columbium-base alloy having good stressrupture strength at high temperatures and consisting essentially of about 20% by weight of tantalum, about 20% by weight of tungsten, and balance essentially columbium.

6. A ductile columbium-base alloy having good stressrupture strength at high temperatures and consisting essentially of about 20% by weight of tantalum, about by weight of molybdenum, and balance essentially columbium.

7. A ductile columbium base-alloy having good stressrupture strength at high temperatures and consisting essentially of from-20 to 40% by weight of -tantalum, from l0 to 20% by weight in the aggregate of at least one ele- ,ment selected from the group consisting of tungsten and .rnolybdenum, the content of molybdenum being a maximum of 15 by weight of the alloy, and balance essentially columbium.

8. A ductile columbium-base alloy having good stressrupture strength at high temperatures and consisting essentially of from 20 to 40% by weight of tantalum, from 10 to 20% by weight in the aggregate of at least one element selected from the group consisting of tungsten and molybdenum, the content of molybdenum being a maximum of 15% by weight of the alloy, from 0.5 to 7% by weight in the aggregate of at least one element selected from the group consisting of hafnium and vanadium, and balance essentially columbium.

References Cited by the Examiner OTHER REFERENCES Gemmell: Transactions of AIME, vol. 215, December 1959, pp. 898-901.

Sims et al.: Transactions of ASM, Preprint No. 70, vol.

DAVID L. RECK, Primary Examiner.

W. C. TOWNSEND, C. N. LOVELL,

Assistant Examiners. 

2. A DUCTILE COLUMBIUM-BASE ALLOY HAVING GOOD STRESSRUPTURE STRENGTH AT HIGH TEMPERATURES AND CONSISTING ESSENTIALLY OF ABOUT 20% BY WEIGHT OF TANTALUM, ABOUT 10% BY WEIGHT OF TUNGSTEN, ABOUT 5% BY WEIGHT OF MOLYBDENUM, ABOUT 5% BY WEIGHT OF HAFNIUM, AND BALANCE ESSENTIALLY COLUMBIUM. 